Studies of information coding in the vertebrate olfactory system have been hindered by the vast number of possible stimuli and the large number of sensory neuron types. In mice there are perhaps as many as 1,000 different classes of olfactory sensory neurons, that differ by the olfactory receptor gene they express. An outstanding issue is the contribution of these olfactory receptor gene products in determining odorant responsiveness of sensory neurons. They hypothesis is that the olfactory receptor solely determines the functional properties of olfactory sensory neurons. The proposed experiments integrate genetic, molecular and physiological approaches to determine 1) if neurons that express the same olfactory receptor gene respond similarly to odorants and 2) if expression of an olfactory receptor gene is necessary and sufficient to confer and operant response phenotype on individual sensory neurons. A significant advance is that odorant sensitivity will be studied in sensory neurons that are homogenous with respect to the olfactory receptor gene they express.